The revolution in high bandwidth applications and the explosive growth of the Internet have created capacity demands that exceed traditional time division multiplexing (TDM) technology. To meet growing demands for bandwidth, Dense Wavelength Division Multiplexing (DWDM) technology has been developed that multiplies the capacity of a single fiber, and enables carriers to adopt optically-based transmission networks that will meet the next generation of bandwidth demand at a significantly lower cost than installing new fiber.
Dense wavelength division multiplexing (DWDM) technology utilizes a composite optical signal carrying multiple information streams, each transmitted on a distinct optical wavelength. The advent of integrated DWDM networks incorporating Network Elements (NE) such as erbium doped fiber amplifiers (EDFA) and optical add/drop multiplexers, (OADM) has enabled the realization of high bandwidth optical networks.
There are several aspects that make the design of DWDM systems unique. The spectrum of DWDM channels may begin to accumulate gain tilt and ripple effects as the signals propagate along a chain of optical amplifiers. Furthermore, each optical amplifier introduces amplified spontaneous emissions (ASE), i.e. noise, into the system which cause a decrease in the signal to noise ratio (OSNR), leading to signal degradation. This is significant as it is the OSNR that is delivered into the photodetector that ultimately determines the Bit Error Rate (BER) performance of a DWDM channel.
The optical noise (ASE) added by every optical amplifier in the network chain, gives rise to a problem because of the existence of gain variations between channels. Some of the gain variations are systematic variations due, for example, to the accumulated gain ripple of optical amplifiers, whereas other variations are random; for example, loss variation of individual components of demux/mux filter structures that will be seen as a loss variation between the different paths through them.
In practice, there is always a tight operating window determined by the receive power level, at which an acceptable OSNR can be achieved, and the transmit level, at which non-linear effects make channel behavior very difficult or impossible to predict. Thus it is very desirable to achieve normalization of wavelength power level within an aggregate in the presence of gain variations.
Metropolitan applications of DWDM technology face special challenges not typically found in long haul point-to-point systems, for two reasons. Firstly, the typical deployment model uses rings to connect OADMs, and consequently individual wavelengths or band of wavelengths travel different distances around the ring between their sources and destinations. By contrast, a typical long haul deployment transmits a complete aggregate of wavelengths between a single source and a single destination point.
Schemes in the past have proposed using power control on the individual wavelengths to keep power levels at the same level. This is expensive, and operationally complex. Other schemes have proposed equalizing OSNR to ensure that each channel is identical, which is ideal for point-to-point transmission but not with Metropolitan (Metro) deployments that use OADMs. Additionally, for Metro deployments, the cost sensitive nature of the market means that the use of additional components simply for power balancing with no functional purpose cannot be justified, For example, full channel breakout for equalization at intermediate points is not a commercially viable solution.
There is a need for a low overhead method and apparatus for automatically balancing optical power levels in DWDM systems so as to minimize the OSNR impairment. This is vital for achieving acceptable system performance. Accordingly it is an object of the invention to provide power balancing apparatus and a technique for delivering each channel within an amplified DWDM system to its receiver with an adequate OSNR to achieve the required system BER.
It is a general objective of the present invention to overcome or significantly mitigate one or more of the aforementioned problems.